Assessing the Efficacy of Anterior Repositioning Splints in the Management of Temporomandibular Disc Displacement: A Systematic Review and Meta-Analysis

Disc displacement (DD) occurs when the cushioning disc of the temporomandibular joint (TMJ), situated between the jawbone and the skull, is out of position. The condition can be of two kinds: disc displacement with reduction (ddwr) and disc displacement without reduction (ddwor). The present systematic review was undertaken to assess the efficacy of anterior repositioning splints (ARS) for ddwr and ddwor. Three online databases were searched for relevant studies using MeSH keywords and Boolean operators. Initial searches of the databases extracted 552 records. Twelve studies equally representing ARS with ddwr and ddwor were selected. No clear difference could be assessed regarding ARS usage in both conditions. The outcome assessed was the improvement in TMJ symptoms. The results suggested that both ddwr and ddwor were effective in improving temporomandibular joint (TMJ) symptoms in patients who received ARS therapy. Efficacy levels ranged from 71-83% and 50-95% for ARS in ddwr and ddwor, respectively; odds ratio (OR) values ranging from 0.30 and 0.36 were obtained for ARS in both therapies, respectively, indicating similar performance levels. Both ddwr and ddwor respond well to the use of ARS; however, more clinical trials are needed to ascertain and validate the role of ARS as a treatment modality in this regard.


Introduction And Background
Temporomandibular joint (TMJ) disorders refer to a range of conditions that affect the joints, muscles, and nerves of the jaw.The temporomandibular joint connects the jawbone to the skull and is responsible for the movement of the jaw when chewing, speaking, and yawning [1].These can be caused by a variety of factors, such as injury to the jaw, bruxism, arthritis, stress, or misalignment of the teeth or jaw [2].The symptoms of TMJ disorders manifest as pain or tenderness in the jaw, clicking sounds during movement of the jaw, difficulty chewing or speaking, tinnitus, vertigo and headaches [3].
There are several treatment modalities available for TMJ disorders, depending on the severity and underlying cause of the condition [4].Pharmacological aid may include over-the-counter pain relievers that help relieve pain and reduce inflammation.Muscle relaxants and tricyclic antidepressants may also be prescribed to help manage symptoms [5].A dental splint or mouthguard can help alleviate symptoms by protecting against clenching or grinding of the teeth.Dental treatments such as bite adjustments or orthodontic treatments may also be recommended to help realign the teeth and jaw.Exercises to strengthen the jaw muscles and improve the range of motion can help alleviate symptoms.Massage therapy, stretching, and injection of botox to the massetric muscle may also be recommended [6].In severe cases, surgery may be necessary to repair or replace the temporomandibular joint.This may involve arthroscopy, a minimally invasive procedure, or open-joint surgery [7].
Another treatment modality is the utilisation of ARS, a type of dental splint that is sometimes used to treat TMJ disorders [8].They are sometimes called "anterior bite planes" or "anterior deprogrammers."An ARS is a custom-made dental appliance to fit over the maxillary or mandibular teeth, and it is designed to move the lower jaw forward and away from the temporomandibular joint [9].By doing so, an ARS can help reduce pressure on the joint, alleviate pain and discomfort, and improve jaw function.An ARS is typically made from acrylic or other durable materials and is worn for a certain period each day, usually several weeks or months.During this time, the patient may be advised to avoid hard or chewy foods and to perform certain exercises or stretches to help improve jaw function [10].
Specifically, an ARS is designed to alleviate pain and discomfort caused by the displacement or damage of the disc that cushions the joint.This can occur as a result of a jaw injury, clenching or grinding of the teeth, misalignment of the teeth or jaw, or other factors.It is intended to reposition the jaw in a way that reduces pressure on the temporomandibular joint, allowing it to heal and function properly [11].It can also help alleviate related symptoms such as headaches, neck pain, and ear pain.In addition to TMJ disorders, an ARS may also be used in certain cases of bruxism or malocclusion [12].
Disc displacement (DD) refers to a condition in which the cushioning disc located between the jawbone and the skull in the TMJ is not correctly aligned [13].This can occur for various reasons, such as trauma or injury to the joint, degenerative joint disease, or long-term grinding or clenching of teeth.There are two types of DD: ddwr and ddwor.In ddwr, the disc moves out of its normal position when the jaw is opened but then returns to its normal position when closed [14].This type of disc displacement is often accompanied by a clicking or popping sound when the jaw is moved, but it may not cause any pain or discomfort.In ddwor, the disc remains out of position when the jaw is opened, and it does not return to its normal position when the jaw is closed.This type of DD can be associated with pain, discomfort, and restricted jaw movement [15].Treatment options may include medication, physical therapy, dental appliances like anterior repositioning splints, or, in some cases, surgery [15].
In both cases, an ARS can be used to help alleviate symptoms and promote healing of the TMJ.For ddwr, the ARS is designed to reposition the jaw to reduce pressure on the joint and helps the disc return to its normal position.For ddwor, the ARS is designed to reposition the jaw to minimise stress on the joint and prevent further damage to the disc [16][17].There are many studies on the use of ARS for TMJ disorders; still, these studies have produced mixed results, and the quality of the evidence may be limited due to small sample sizes or variations in study design [16][17][18].Hence, this review was undertaken to synthesise evidence on the effectiveness of anterior repositioning splints for disc displacement conditions.

Review Framework and Registration
The Preferred Reporting Items for Systematic reviews and Meta-Analyses (PRISMA) protocol [19] was followed for the purpose of guiding the search strategy and its implementation.The PICO strategy employed for the review was-Population: patients diagnosed with temporomandibular joint disc displacement; Intervention: it included the application of anterior repositioning splints (ARS) therapy; Comparison: no treatment, other types of TMJ treatment, or a placebo intervention; and Outcome: Improvement in TMJ symptoms, including pain, clicking, jaw function, quality of life, and changes in disc position as measured by various imaging and diagnostic tests.
Using this PICO strategy, the systematic review and meta-analysis aimed to answer questions such as: "What is the overall effectiveness of anterior repositioning splints for treating TMJ disorders caused by disc displacement with and without reduction?"and "What are the potential benefits and harms of this treatment approach compared to other interventions or no treatment?"

Search Protocol Across Databases
The databases of PubMed, Web of Sciences, and Scopus were scoured for the identification and selection of relevant studies, the strategy of which is as follows.PubMed: ("temporomandibular joint disorders[Mesh]) OR ("temporomandibular joint disc displacement" [Me AND ("splints[Mesh] OR "orthotic devices" AND ("repositioning[Subheading] OR ading] OR "anterior" [ NOT ("surgery[Mesh] OR "drug therapy[Mesh] OR "physical therapy]. Web of Science: TS=("anterior repositioning splints" OR "anterior repositioning appliances" OR "anterior repositioning devices" OR "anterior repositioning orthotics") AND TS=("temporomandibular joint disorders" OR "temporomandibular joint disc displacement") NOT TS=("surgery" OR "drug therapy" OR "physical therapy").
Scopus: TITLE-ABS-KEY (anterior repositioning splints" OR "anterior repositioning appliances" OR "anterior repositioning devices" OR "anterior repositioning orthotics") AND TITLE-ABS-KEY("temporomandibular joint disorders" OR "temporomandibular joint disc displacement") NOT TITLE-ABS-KEY (surgery" OR "drug therapy" OR "physical therapy") In all three databases, the search terms were combined using boolean operators (AND, OR, NOT) to refine the search and identify relevant studies.MeSH keywords were used in PubMed, while title/abstract keywords were used in Web of Science and Scopus.No time frame limitation was applied to the search.

Eligibility Criteria
Studies that included adult patients with clinically diagnosed TMJ disorders and used ARS as the primary treatment modality or as an adjunct to other treatments were included, with no restriction on timeline.Case reports or case series, reviews, commentaries, and editorials were excluded.Studies that focused on other TMJ treatment modalities, such as pharmacotherapy, physical therapy, or surgical intervention, were excluded.Studies that did not report relevant outcomes such as pain, jaw function, or disc position were also excluded.

Data Extraction
The search was conducted by two independent reviewers who screened the titles and abstracts of identified studies for inclusion, followed by a full-text review of potentially eligible studies.Discrepancies between reviewers were resolved through discussion or a third reviewer.Data were extracted from the included studies by the same reviewers using a standardized form.Variables extracted from each study were study ID, publication year, type of disc displacement, sample size, age range, gender ratio, diagnostic technique, follow-up, and inferences.

Risk of Bias Assessment
The Risk of Bias-2 (ROB-2) [20] was used to assess the methodological quality of the studies included.The same reviewers assessed each included study using the tool.The tool assessed the risk of bias across six principal domains, and for each domain, the reviewers assessed the risk of bias as low, high, or unclear.

Protocol for Meta-Analysis
Review Manager 5.4 (RevMan Web, The Cochrane Collaboration, UK) was used for statistical analysis.Pooled results across studies using fixed-effects models in terms of the OR of ddwr and ddwor at a 95% confidence interval (CI) were tabulated.Forest plots were generated using the software to visualise the results of the meta-analyses and identify any heterogeneity or inconsistency across studies.
The methodological assessment of the quality of the included studies showed moderate to good quality studies, as shown in Figure 2. The OR of 0.30 (0.22, 0.40) (Figure 3) represents the summary effect estimate of the impact of ARS usage in studies of cases representing ddwr.The forest plot displaying the overall positive vs. overall negative impact of ARS usage indicates that the overall effect is statistically significant, as evidenced by the test for overall effect with Z = 7.98 (p < 0.00001).This means that the observed odds ratio of 0.30 is unlikely to have occurred by chance, and we can conclude that there is a true effect of ARS usage in studies of cases representing ddwr.The heterogeneity test indicates that there is no significant heterogeneity among the studies.This suggests that the studies are relatively consistent in their estimates of the effect of ARS usage and that the summary odds ratio is a reliable estimate of the true effect size.Overall, this indicates that ARS usage has a significant overall positive impact in studies of cases representing ddwr, with an OR of 0.30 (0.22, 0.40).Figure 4 shows an OR of 0.36 (0.27, 0.49) representing the summary effect estimate of the impact of ARS usage in studies of cases representing ddwor.The forest plot displaying the overall positive vs. overall negative impact of ARS usage indicates that the overall effect is statistically significant, as evidenced by the test for overall effect with Z = 6.53 (p < 0.00001).This means that the observed odds ratio of 0.36 is unlikely to have occurred by chance, and we can conclude that there is a true effect of ARS usage in studies of cases representing ddwor.No significant heterogeneity among the studies was reported, suggesting consistency in the effect estimate of ARS usage.The therapy demonstrated a significant overall positive impact in studies of cases representing ddwor, with an OR of 0.36 (0.27, 0.49).Overall, the synthesised evidence suggests that ARS was effective in reducing TMD symptoms in both conditions of ddwr and ddwor.

Discussion
The review analysed a range of different study designs, encompassing both retrospective and prospective studies, as well as observational and comparative trials.The findings from this review suggest that ARS usage has a significant overall positive impact on both types of conditions, with summary effect estimates of 0.30 and 0.36 for ddwr and ddwor, respectively.One potential gap in the literature that this study addresses is the lack of systematic reviews and meta-analyses that have examined the impact of ARS usage in the treatment of ddwr and ddwor.By synthesising the available evidence from multiple studies, this review provides a more comprehensive understanding of the potential benefits of ARS usage in these conditions.The findings of this review have important implications for clinical practice.If confirmed by further research, these results suggest that ARS usage may be an effective treatment option for individuals with ddwr and ddwor.This could potentially improve outcomes and quality of life for individuals with these conditions.Additionally, these findings could help inform treatment decisions for clinicians working with individuals with these conditions.
Protruding the mandible is the beginning point of ARS therapy, and as a result, the procedure alters the interaction between the articular disc and condyle [25].In the early stages of application, protruding the mandible causes the condyle to shift forward and "catch" the displaced disc [28].This corrects the connection between the disc and the condyle and makes it easier for the tissue to adapt or heal.The disc-condyle complex is put back in its proper position as the mandible returns to its initial position after therapy is finished by gradually grinding the splint [23].
The concept of restoration, according to some medical professionals, is to move the condyle inferoanteriorly and let the disc reposition itself.According to a study that used MRI to study the discs of patients with ddwr, 41.9% of the discs that had been recovered at the edge-to-edge position were displaced once more during the process of backing to the least protrusive position [33].The forward and downward condyle movements that fix the disc in a favourable position and keep it from shifting again during the mouth-closing process provided strong support for the hypothesis of another study, according to this finding [33].A healthy disccondyle connection can lessen the disc's collision, rebound, and release, which can stop chronic disc injury from occurring during abnormal movements [34].Stress distribution also varies as internal structure changes.According to some studies, reducing unfavourable loading was a key factor in symptom relief [27,35].Elastic tissues and articular cartilage may deteriorate as a consequence of pressure that results from stress that is concentrated in some areas [31].
Hard tissues are impacted by changes in stress distribution and undergo an adaptive shift as a result.Hollender and Lindahl [36] made the initial discovery of the "double contour" pictures while remodelling condylar fractures.The images visible in X-ray, CT, and MRI scans are typically recognised as significant indicators of development and remodelling.Another study found no significant difference in the incidence rate between teenage and adult groups and also found that patients with DD who received ARS therapy showed "double contour" images, which only appeared in the joints with improved internal structure [37].
The articular disc can become damaged by concentrated tension.As per the study of Machon et al. [38], anterior DD frequently results in distal puncture.The anterior displacement of the disc increased the maximum compressive stress to 14.6 times higher than its average score, with the maximum tensile stress reaching 1.43 MPa at the intermedial zone, which is in close proximity to tensile stress failure [39].Tanaka et al. [40] further reinforced this observation.Anterior DD was associated with a five-fold rise in the shear stresses in the bilaminar zone.The bilaminar zone is an area of loose connective tissues, which is heavily vascularized and innervated and is referred to as the posterior attachment [36,40].The only structure capable of retracting the disc posteriorly on the condyle is the superior retrodiscal lamina in the bilaminar zone [41].
Abnormal stress in this region may not only cause pain but may also decrease the elasticity of this region, leading to laceration or perforation.These findings suggest that compressing the disc may help decrease the concentration of local stress, thereby avoiding disc damage and relieving pain [42].Experimental displacement using two different methods is required to determine how much ARS can alter the stress distribution.First off, after the splint is removed, an adaptive shift in the condyle encourages disc stabilisation and enhances the disc-condyle relationship.Second, among children and adolescents with TMD, ddwor is significantly associated with TMJ disorders [43].TMD conditions can prevent the condyle from growing normally and result in malformation [44], but they do not have a reliable cure.According to Lei et al.'s research [45], ARS can help juvenile patients with early-stage TMJ degenerative joint disease (DJD) repair and regenerate their condyles and avert a poor prognosis for DD.These conclusions were supported by laboratory tests.Mandible protrusion has been shown by a couple of papers [46][47] to raise growth factor, hasten mesenchymal cell differentiation into chondrocytes, and increase chondrogenesis and osteogenesis in mandibular condyles.According to another study [48], the mandible protrusion may encourage cartilage development modification by activating genetic signalling pathways in the TMJ.

Study limitations
A limitation of this review was the small number of studies included.Although the review analysed a range of different study designs, only 12 studies met the inclusion criteria.This relatively small sample size may limit the generalizability of the findings, and further research is needed to confirm these results.Another limitation is the potential for publication bias.As with any meta-analysis, there is a risk that studies with negative findings may be underrepresented in the analysis due to publication bias.This could potentially overestimate the impact of ARS usage in the treatment of ddwr and ddwor.Moreover, the study did not examine potential adverse effects or long-term outcomes associated with ARS usage.This information could be important for clinicians considering ARS as a treatment option for individuals with ddwr and ddwor

Conclusions
In conclusion, this review analysed a total of 12 studies that investigated the impact of ARS usage in cases of ddwr and ddwor.Overall, a positive response to the usage of ARS was seen in all studies, with varying levels of percentage, and the statistical analysis indicates that ARS usage has a significant overall positive impact in studies of cases representing both ddwr and ddwor.The summary effect estimates for the impact of ARS usage in studies of cases representing ddwr and ddwor were 0.30 and 0.36, respectively, and both were statistically significant.The studies were relatively consistent in their estimates of the effect of ARS usage, and the summary odds ratios are likely to be reliable estimates of the true effect sizes.Therefore, these findings suggest that ARS usage can be a valuable treatment option for both ddwr and ddwor.

FIGURE 1 :
FIGURE 1: Framework representing the study selection protocol using PRISMA PRISMA: Preferred Reporting Items for Systematic Reviews and Meta-Analyses

FIGURE 3 :
FIGURE 3: OR representation on forest plot displaying the impact of ARS usage in studies of cases representing ddwr OR: Odds ratio; ARS: Anterior repositioning splints; ddwr: Disc displacement with reduction

FIGURE 4 :
FIGURE 4: OR representation on forest plot displaying the impact of ARS usage in studies of cases representing ddwor OR: Odds ratio; ARS: Anterior repositioning splints Pihut M et al [31] = Pihut et al. (2013)[31]

TABLE 1 : Variables analysed in the included papers
Tables 2-3 showed the impact of ARS in ddwr and ddwor cases, respectively, alongside other information pertaining to the protocol employed and the follow-up periods in them.

TABLE 3 : Details pertaining to studies where ddwor was observed
ddwor: Disc displacement without reduction